Earth-boring tools for forming wellbores in subterranean earth formations generally include a plurality of cutting elements secured to a body. For example, fixed-cutter earth-boring rotary drill bits (also referred to as “drag bits”) include a plurality of cutting elements that are fixedly attached to a bit body of the drill bit. Similarly, roller cone earth-boring rotary drill bits may include cones that are mounted on bearing pins extending from legs of a bit body such that each cone is capable of rotating about the bearing pin on which it is mounted. A plurality of cutting elements may be mounted to each cone of the drill bit.
The cutting elements used in such earth-boring tools often include polycrystalline diamond compact (often referred to as “PDC”) cutting elements, which are cutting elements that include cutting faces of a polycrystalline diamond material. Such polycrystalline diamond cutting elements are formed by sintering and bonding together relatively small diamond grains or crystals with diamond-to-diamond bonds under conditions of high temperature and high pressure in the presence of a catalyst (such as, for example, Group VIIIA metals including, by way of example, cobalt, iron, nickel, or alloys and mixtures thereof) to form a layer or “table” of polycrystalline diamond material on a cutting element substrate. These processes are often referred to as high temperature/high pressure (or “HTHP”) processes. The cutting element substrate may comprise a cermet material (i.e., a ceramic-metal composite material) such as, for example, cobalt-cemented tungsten carbide. In such instances, the cobalt (or other catalyst material) in the cutting element substrate may be swept into the diamond crystals during sintering and serve as the catalyst material for forming the diamond table from the diamond crystals. In other methods, powdered catalyst material may be mixed with the diamond crystals prior to sintering the crystals together in an HTHP process.
Upon formation of a diamond table using an HTHP process, catalyst material may remain in interstitial spaces between the crystals of diamond in the resulting polycrystalline diamond table. The presence of the catalyst material in the diamond table may contribute to thermal damage in the diamond table when the cutting element is heated during use due to friction at the contact point between the cutting element and the formation. Accordingly, the polycrystalline diamond cutting element may be formed by leaching the catalyst material (e.g., cobalt) out from interstitial spaces between the diamond crystals in the diamond table using, for example, an acid or combination of acids, e.g., aqua regia. Substantially all of the catalyst material may be removed from the diamond table, or catalyst material may be removed from only a portion thereof, for example, from the cutting face, from the side of the diamond table, or both, to a desired depth.
PDC cutters are typically cylindrical in shape and have a cutting edge at the periphery of the cutting face for engaging a subterranean formation. Over time, the cutting edge becomes dull. As the cutting edge dulls, the surface area in which the cutting edge of the PDC cutter engages the formation increases due to the formation of a so-called wear flat or wear scar extending into the side wall of the diamond table. As the surface area of the diamond table engaging the formation increases, more friction-induced heat is generated between the formation and the diamond table in the area of the cutting edge. Additionally, as the cutting edge dulls, the downward force or weight on the bit (WOB) must be increased to maintain the same rate of penetration (ROP) as a sharp cutting edge. Consequently, the increase in friction-induced heat and downward force may cause chipping, spalling, cracking, or delamination of the PDC cutter due to a mismatch in coefficient of thermal expansion between the diamond crystals and the catalyst material. In addition, at temperatures of about 750° C. and above, presence of the catalyst material may cause so-called back-graphitization of the diamond crystals into elemental carbon.
Accordingly, there remains a need in the art for cutting elements that increase the durability as well as the cutting efficiency of the cutter.